The present invention relates to methods of operating and control systems for refrigeration systems and, more particularly, to methods of operating and control systems for surge control devices, such as compressor inlet guide vanes, in dual centrifugal vapor compression refrigeration systems whereby when one compressor begins to operate in a surge condition, the other compressor's guide vanes are closed.
Generally, refrigeration systems include an evaporator or cooler/chiller, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evaporator. The heat transfer fluid chilled in the tubing in the evaporator is normally water or glycol which is circulated to a remote location to satisfy a refrigeration load. The refrigerant in the evaporator evaporates as it absorbs heat from the heat transfer fluid flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.
To maximize operating efficiency, it is desirable to match the amount of work done by the compressor to the work needed to satisfy the refrigeration load placed on the refrigeration system. Commonly, this is done by capacity control means which adjust the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device such as guide vanes which are positioned between the compressor and the evaporator which move between a fully open and a fully closed position in response to the temperature of the chilled heat transfer fluid leaving the coil in the evaporator. When the evaporator chilled heat transfer fluid temperature falls, indicating a reduction in refrigeration load on the refrigeration system, the guide vanes move toward their closed position, decreasing the amount of refrigerant vapor flowing through the compressor. This decreases the amount of work that must be done by the compressor thereby decreasing the amount of energy needed to operate the refrigeration system. At the same time, this has the effect of increasing the temperature of the chilled heat transfer fluid leaving the evaporator. In contrast, when the temperature of the leaving chilled heat transfer fluid rises, indicating an increase in load on the refrigeration system, the guide vanes move toward their fully open position. This increases the amount of vapor flowing through the compressor and the compressor does more work thereby decreasing the temperature of the chilled heat transfer fluid leaving the evaporator and allowing the refrigeration system to respond to the increased refrigeration load. In this manner, the compressor operates to maintain the temperature of the chilled heat transfer fluid leaving the evaporator at, or within a certain range of, a set point temperature.
Many different capacity control systems are known for controlling a refrigeration system in the manner described above. For example, one such control system, adjusts a capacity control device in a refrigeration system as a function of the deviation of leaving evaporator chilled water temperature from a desired set point temperature. When the evaporator chilled water temperature deviates from the selected set point temperature by a predetermined amount the capacity control device is continuously adjusted by an actuator which is continuously energized by a stream of electrical pulses supplied to the actuator.
However, with dual centrifugal compressor systems, where one compressor is designated "lead" and the other compressor is designated "lag", the compressors are generally controlled by monitoring the percent of full load electrical motor current of the lead compressor and by adjusting the guide vanes of the lag compressor, either open or closed, until the lag percent of full load motor current matches the lead compressor percent of full load motor current. When the lead compressor surges with this operating scheme the surge condition is generally alleviated, since, when the lead compressor surges, its motor current drops severely. When the lag compressor senses this it closes its guide vanes in an attempt to match the lead compressor's percent motor current. As a result, the refrigeration system's capacity is temporarily reduced, and the evaporator and condenser pressures approach each other, but, since the surge is caused by the system operating at too high a pressure difference, the surge is stopped.
When the lag compressor is the one that surges, however, the surge condition cannot be stopped with the prior art control scheme. When the lag compressor surges, causing machine capacity to drop off, the lead compressor whose guide vanes are controlled in response to leaving chilled water responds by opening its guide vanes in an attempt to restore system capacity. Accordingly, the cooler and condenser pressures do not approach each other, and the lag compressor would continue to surge indefinitely.
Thus, there exists a need to develop lead/lag control techniques for multiple centrifugal compressor machines, when the compressors are connected in parallel, which minimizes the disadvantages of controlling capacity in response to surging of the lag compressor by adjusting the guide vane of the lead compressor.